It's not all photos. A pic at 1/5s f3.5 iso1600 is fine, another pic identical but at 1/20s is not. So it could just be the under-exposed ones that are the issue and the bands are a natural consequence of demanding more than the sensor can manage (although it begs the question why the camera didn't auto-ISO above 1600 - although at that point it may have been set to a max of 1600).

However, I have just taken another picture with the Panasonic 14-42 X at iso 5000 (1/3s and f3.5) and that same issue has occurred. The reason for trying again is to avoid interference from my PC (which was nearby in the first photo, but not in the one I just tried).

Sadly I'm waiting for Lightroom to update before I look at any raw images, so can't check whether it's occurring with those.

~~Answer~~

Ok, I've been writing this note for 45 minutes now. In that time I've had a play with the Noise Reduction (NR) and the Noise Filter (NF) settings. Eight photos, all of the same scene (shot handheld, so minor differences), a dark room with lights behind the window, all at 1/3s f3.5.

With NR set to "Auto", I sometimes get those bands, and sometimes don't.
With NR set to "Off", I always get those bands, whatever NF is set to.
With NR set to "On", I never get those bands, whatever NF is set to.

This basically suggests that forcing NR to 'on' will remove those artefacts from JPGs. I'll probably find out that this is all JPG artefacts and no issue at all once I start using the RAW images, so I'll experiment again once I can process the RAW images. erm. ORF images.

Incidentally, NR 'On' and NF 'Low' gave a different interference pattern that's not visible in the JPG at full size, but which my graphics viewer of choice created as part of rendering at screen size (rather than full size).

Finally, in a very subjective interpretation of which settings work best, I prefer NR 'On' and NF 'Off' - but with NR 'On' and NF 'High' as second favourite, so I suspect other factors are at play here

However, this impromptu test has made one thing clear: If you're doing low-light photography, set Noise Reduction to 'On'. Note : If you're not doing low-light photography, it may be better to leave NR on 'Auto', as the manual notes that it "requires about twice the time needed to record the image". However, I was still getting the 7-8 fps shooting in RAW so in real use it's not going to impact me.

Anybody had similar or divergent results? Is this related to the (lack of) AA filter? Am I just weird?

I've noticed a different type of artefact in a couple of EPL5 Jpeg images - It's like horizontal and vertical pit-like marks in some plain colour areas.
The artefacts are not there in the RAW (which is supported now in Lightroom 4.3).

The E-PL5 does have an AA filter on the sensor and the sensor is the exactly the same as in the E-M5. The rumour that it didn't got around because an Olympus employee in Europe made a mistake and said it didn't. That statement haas since been retracted.

What lens? The 20mm F/1.7 has a banding problem at high ISO with the E-M5 so I'd suspect that it would also with the E-PL5.

Apart from that, banding can occur in dark areas because there isn't enough level information recorded in the dark areas to provide for a smooth tonal gradation (digital cameras record most of their data in the highlight range and progressively less in darker areas), and noise is also always worst in dark areas.

Apart from that, banding can occur in dark areas because there isn't enough level information recorded in the dark areas to provide for a smooth tonal gradation

Click to expand...

I'd expected high(er) levels of noise due to that, but not the strange regular horizontal bands. However, it does seem to be a noise influenced effect, given that forcing Noise Reduction 'on' removes the issue.

I'd expected high(er) levels of noise due to that, but not the strange regular horizontal bands. However, it does seem to be a noise influenced effect, given that forcing Noise Reduction 'on' removes the issue.

Click to expand...

Many CMOS sensors do seem to show banding in certain situations when the ISO is pushed high. I've seen it on my E-M5 numerous times at 6400 (even when not using the 20/1.7 lens) and on my D700 before that at ISO 25600 (the E-PM1 will do it at 1600 sometimes too). It looks like Olympus has made some attempt to clean it up in JPEG, which is nice.

Another 600 photos in, and something new learned:
- this issue does appear in the RAW (erm, .ORF) files
- if you use multiple exposures (e.g. the 8fps setting) then "Noise Reduction" is set to 'Off' and you are still in danger of getting the banding.

I got lucky and lost no nice photos to this, but I've now switched back to single shot as my default.

Another 600 photos in, and something new learned:
- this issue does appear in the RAW (erm, .ORF) files
- if you use multiple exposures (e.g. the 8fps setting) then "Noise Reduction" is set to 'Off' and you are still in danger of getting the banding.

I got lucky and lost no nice photos to this, but I've now switched back to single shot as my default.

Click to expand...

That's really bizarre. For your shots, NR = Auto should have the same impact as NR = No - it doesn't kick in for exposures shorter than 4 seconds.

I think you are misunderstanding Cederic. He keeps NR on at all times at high ISO's as this eliminates banding (or at least most of it). But it seems that the high sequence mode will switch NR off regardless of your NR settings.